Can you please specify which drives you are considering? Typically with a cache size difference as large this the drives are of significantly different capacity. The greater the areal data density on the platters, the higher the sequential read/write performance. Random I/O varies from model to model and can only be determined by benchmarks.
Also, as 15K drives tend to be low capacity (less than 600GB), a RAID is often used. The cache of your RAID controller will matter more the the cache of the individual drives. If you do not need enough space to require an array, consider using SSD(s). SSDs excel in random I/O which is handy for databases.